Solid state light sources such as light emitting diodes (LEDs) are among the most efficient light sources available today. LEDs provide longer lifetime, higher photon flux efficacy, lower operating voltage, narrow-band light emission, and flexibility in terms of assembly compared to conventional light sources.
Commonly, III-V semiconductor materials are used to provide high-brightness light emitting devices operating in the ultraviolet, visible or infrared regions of the electromagnetic spectrum. The materials used include for example binary, ternary and quaternary alloys of gallium, aluminum, indium, nitrogen, phosphorus and arsenic.
Gallium nitride (GaN) LEDs have recently attracted much attention as efficient light sources, where the combination of GaN with In (InGaN) or Al (AlGaN) further allows for tailoring of the emission energy of the photons emitted by the LED as the band gap of the semiconductor alloy is dependent on ratio of In and/or Al to GaN offering LEDs with colors ranging from red to blue. The GaN based LEDs are therefore successfully used in solid state lighting applications such as illumination, traffic lighting, indoor/outdoor displays, and backlighting electronic displays.
High quality performance of LEDs requires high brightness as well as efficient heat dissipation. Electrical power that is not converted into light emission from the LED converts into heat that must be conducted from the LED to its surroundings, since excess heat in the active region of the LED reduces quantum efficiency and thereby light output. Hence attention must be paid to the thermal architecture of the LEDs as well as to its electronic structure.
A flip-chip (FC) GaN LED die commonly comprises a substrate, said substrate is usually sapphire a low loss transparent material, upon which epitaxially is grown an n-type GaN layer (or layers), a GaN based active region, and a p-type GaN layer (or layers). In FC LED dies, light generated in the active region is extracted from the side of the n-type GaN layer and substrate material. After epitaxial growth, it is required, for biasing the LED, to form proper electrical contacts at the n-type GaN layer, i.e., cathode electrode, and at the p-type GaN layer, i.e. anode electrode. Both p-type GaN and n-type GaN metal contacts are highly reflective in order to redirect the generated light into the sapphire side. Both n- and p-layers are usually electrically contacted from the side of the p-layer. Thus, in order to access the n-layer, openings may be formed by etching away the p-layers and the active region. The n-contact layer electrically contacts the n-type GaN layer through these openings and is usually designed to uniformly distribute the current though the n-type GaN layer, which acts as a lateral current spreading layer. The n-contact layer design aims at avoiding current crowding regions in the active region with a minimum required area of said etched openings.
The LED die is typically attached to a submount, where said anode and cathode electrodes in the die are electrically contacted to metal layers in the submount. The attachment to the submount may e.g. be made by means of stud-bumps.
After die attachment, and in order to improve light extraction, the substrate may be removed by laser-assisted lift-off. Then, the exposed n-type GaN epi surface is roughened by electrochemical etching. The resulting device structure is usually referred to as thin-film-flip-chip (TFFC) LED.
Both FC and TFFC LED die structures commonly consist of a layer stack formed by the following basic elements: GaN layers (n-type GaN layer, active GaN region [typically InGaN] and p-type GaN layer); p-contact layer; at least one dielectric layer to electrically isolate the anode and cathode electrodes; n-contact metal layer; and bonding layer to electrically and mechanically attach die onto a submount or printed circuit board (PCB).
It is generally desirable that this basic combination of layers in the LED die stack can optimally perform the following essential functions: i) lateral current spreading to insure uniform current distribution throughout the active region; ii) lateral thermal spreading to minimize hot-spots and thermal resistance; iii) die interconnection with submount and/or PCB (bonding layer); iv) mechanical stability, particularly in case of TFFC; v) mirror reflection for light extraction; vi) electrical isolation; and vii) metal-semiconductor electrical contact.
As such, each of the layers of the die stack must have one or more of the previous functions, i.e. each layer is referred to as a functional layer, e.g. functional layer p-contact layer has functions v) and vii).
Existing FC die architectures generally present performance limitations in one or more of the seven previously listed essential functions. For example, the stud-bumps interconnect approach (related to functions iii and iv) can confront serious heat-sinking limitations due to the reduced metal interconnect area. This may particularly become critical in case of standard TFFC due to the lack of thermal spreading layers (function ii). Solutions for improved thermal performance has been disclosed but often is the architecture detriment of the die interconnection with submount and/or PCB, which becomes limited in terms of lacking compatibility with SMD technologies. For these solutions also current injection into the n-type GaN layer to minimize current crowding (function i) may be limited.
The multilayer proposal of US20050067624 A1 offers great flexibility to cope with current injection into the n-type GaN layer to minimize current crowding, but this is done by compromising functionally related to either lateral thermal spreading to minimize hot-spots and thermal resistance or die interconnection with submount and/or PCB.
Accordingly, there is a need for a LED die structure wherein functions i)-vii) may be optimized without compromising on another one of the essential functions. This would enable a large degree of freedom in the optimization of the current distribution, thermal spreading and interconnect pad layout of the LED die resulting in improved brightness and/or easier mounting of the LED die to the submount and/or PCB.